-LRB- CNN -RRB- The sun had n't risen at Garissa University College . Most students slept in their beds . A few had woken up to head to early morning Christian prayers .

Then the terror began .

It started with an explosion and gunshots around 5:30 a.m. Thursday -LRB- 10:30 p.m. ET Wednesday -RRB- at the Kenyan school 's front gates . The attackers continued to fire as they stalked through campus , with the Red Cross saying they stopped at a girls ' dormitory .

At one point , they burst into a room where Christians had gathered and took hostages , said lecturer Joel Ayora . A student in the room told Alex Kubasu , a reporter with CNN affiliate Citizen TV , that the terrorists sprayed bullets indiscriminately , striking his thigh .

`` Then they proceeded to the hostels , '' Ayora told CNN , referring to the university dorm , `` shooting anybody they came across -- except their fellows , the Muslims . ''

According to AFP , the gunmen separated the students by religion and allowed Muslims to leave . This would be consistent with the past practices of Al-Shabaab , the Somalia-based terror group that 's claimed responsibility for the attack . That 's what Al-Shabaab did in a December raid on a quarry in the Kenyan village of Kormey , near the Somali border , that ended with at least 36 killed .

Whatever their religion , hundreds of students managed to escape , said Dennis Okari , a reporter with CNN affiliate NTV .

Some ran . Some crawled . All feared for their lives .

And they were the lucky ones .

It was n't until about 15 hours after the attack began that the explosions and gunfire around Garissa finally ended . Interior Ministry Secretary Joseph Nkaissery announced that four terrorists were killed and the operation had ended `` successfully . ''

By then , 147 people were dead and plans were `` underway to evacuate students and other affected persons , '' the Kenya National Disaster Operation Centre tweeted .

`` It is a very sad day for Kenya , '' Nkaissery said .

Some of those students , at one point or another , were believed to be hostages in one of the four dormitories on campus .

Yet others like Ayora managed to find safety , in his case at the staff residence . Others escaped the college 's grounds . Okari , for instance , told CNN that he saw about 200 people sitting in the scorching mid-90-degree heat Thursday in a corner of a Garissa airstrip , surrounded by military officials .

One student , Japhet Mwala , recalled to AFP how `` everyone started running '' after hearing blasts and gunshots .

`` There were those who were not able to leave the hostels where the gunmen headed and started firing , '' Mwala said . `` I am lucky to be alive , because I jumped through the fence with other students . ''

Rosalind Mugambi fled into a nearby field with `` bullets following us . '' She escaped unscathed , but a few of her friends did not .

`` We saw some bloodstains , '' Mugambi told AFP , `` and they were shot . ''

Such terrorism , sadly , is n't new to the people of eastern Kenya . And that 's largely thanks to Al-Shabaab .

While it is based in Somalia , the militant group has made bloody incursions into neighboring Kenya -- most notoriously a 2013 siege of Nairobi 's upscale Westgate Mall that left at least 67 dead .

A primary aim of such attacks is thought to be sowing terror . There 's also a religious undercurrent , given Al-Shabaab 's once and possibly future goal of creating a fundamentalist Islamic state , according to the Council on Foreign Relations . Needless to say , that aim is n't shared in most places -- particularly in Kenya , where more than 80 % of citizens are Christian .

Such violence is more likely in towns like Garissa , given its location 90 miles west of the Somali border .

What makes Thursday 's attack different is what was targeted -- an institute of higher learning , presumably filled with innocent students with no direct part in the Kenyan and allied governments ' fight against Al-Shabaab . Such an attack is more reminiscent of what other Islamist extremist groups , the Pakistani Taliban and Boko Haram , did in respectively attacking a school in Peshawar , Pakistan , and kidnapping more than 200 girls at a school in Chibok , Nigeria .

Garissa University College began in the 1990s as Garissa Teacher Training College before evolving , by government decree , into its current form as a degree-granting institution in 2011 .

As of this spring , the school had schools of education , information science , and arts and social sciences . And it was growing , with its staff of 75 set to expand to 200 by year 's end , according to the college 's website .

It 's too early to tell what the future now holds for Garissa University College . But Kenyan President Uhuru Kenyatta on Thursday urged people around the African nation `` to stay calm as we resolve this matter . ''

`` This is a moment , '' Kenyatta said , `` for everyone throughout the country to be vigilant as we continue to confront and defeat our enemies . ''

CNN 's Josh Levs contributed to this report .

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Kenyan agency : 147 dead , plans underway to evacuate students and others

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Garissa University College students wake to explosions and gunfire

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Reports : Gunmen storm the Kenyan school , attacking Christians and letting Muslims go